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125 Gallon Tank Dimensions

A standard 125 gallon tank is about 72 × 18 × 22 in (183 × 46 × 56 cm) on the outside, holds about 102–106 gallons (387–402 litres) of water once set up, and weighs about 1,160–1,210 lb (525–550 kg) filled. Exact sizes vary by brand: Aqueon lists 72½ × 18½ × 23⅛ in and Marineland 729⁄16 × 1823⁄32 × 22⅛ in. The water and weight figures assume 2 inches of sand and water 1 inch below the rim.

125 gallon tank calculator

Drawn to scale from your numbers: sand at the bottom, water up to your fill line.

Start from a real tank
Outside dimensions
Units
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Glass, tank weight and rock
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106.3US gal of water

Once set up. 134.3 gal by outside dimensions · 117.1 gal before substrate

Total weight on the stand estimate
1,208 lb
Water
402 litres
Footprint
9.3 sq ft
Load on that footprint
130 lb/sq ft
Heater rule of thumb
319–532 W
Filter flow rule of thumb
425–638 gph
Substrate needed
145 lb
Water weight
887 lb
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125 gallon tank dimensions: 72 × 18 × 22 inches (183 × 46 × 56 cm), length × width × height, with Aqueon and Marineland sizes
125 gallon tank dimensions, drawn to scale: 72 × 18 × 22 in outside. Makers' exact sizes differ by a fraction of an inch; both are compared below. Free to reuse with credit (CC BY 4.0).

Setting up a 125?

Not every 125 gallon is the same size

The “125 gallon” label is a class of tank, not an exact measurement. Here is what each source actually lists:

SourceL × W × H (in)EmptyFull
Aqueon size chart (125)72½ × 18½ × 23⅛177 lb1,400 lb
Marineland ML10125 (dealer listing)729⁄16 × 1823⁄32 × 22⅛160 lb1,160 lb

Checked 29 Sep 2026. Marineland lists 1,160 lb full, below the label 125 gallons × 8.34 lb plus the empty tank (1,202 lb). Aqueon's 1,400 lb is more than even the whole outside volume filled with water (1,297 lb); its chart notes that filled weights vary with substrate and decor. Neither includes the stand. The makers' "full" weights don't say how full; our set-up estimate further down uses the water left after 2 in of sand and a gap below the rim, plus the sand, so it can come out lower. Size your stand for the higher figure.

Six-foot tanks

The 125 is the entry point to 6 ft tanks. The two standard sizes on a 72 × 18 footprint share a stand; the 180 is 6 in deeper and needs its own:

TankL × W × H (in)Outside volume
125 gallon72½ × 18½ × 23⅛134.3 gal
150 gallon72½ × 18½ × 29½171.3 gal
180 gallon (different stand)72 × 24 × 24179.5 gal

Aqueon size chart (125, 150); Marineland dealer listing (180, nominal size).

125 vs 120: long or deep?

They hold similar water but have very different shapes:

TankL × W × H (in)FootprintWater, 2 in sand
125 (Aqueon)72½ × 18½ × 23⅛9.3 sq ft106.3 gal
120 (Marineland)48⅜ × 24⅜ × 258.2 sq ft103.1 gal
  • 125: 6 ft of swimming length for large, active fish. It needs a 6 ft wall and a 72 in stand and light.
  • 120: 4 ft long but 2 ft front to back, which gives aquascapes real depth and fits rooms without a 6 ft wall. It puts more weight on each square foot: about 142 lb against the 125's 130.

Water estimates use 1/2 in glass for both (an estimate; check your tank).

How much water a 125 gallon really holds

Multiply the outside dimensions of an Aqueon 125 and you get 134.3 gallons. You never get that much water, because of three things:

  1. Glass. With about 1/2 in glass (an estimate; check your tank), the inside is roughly 71½ × 17½ in.
  2. The gap at the top. Most people keep the water about an inch below the rim.
  3. Substrate. Two inches of sand takes up about 41 litres of space.

That leaves about 117.1 gallons with a bare bottom and 106.3 gallons with 2 in of sand. Use the real figure, not the label, when you dose medication or water conditioner.

Weight and stand

A set-up Aqueon 125 comes to about 1,208 lb: 177 lb of glass, about 887 lb of water and about 145 lb of sand, before rock and the stand itself. That sits on 9.3 square feet.

  • A set-up 125 weighs about 1,208 lb before rock; Aqueon lists 1,400 lb. That's a lot for one spot on a floor.
  • Run the 6 ft length across the floor joists, not along one or two of them, and keep it close to a load-bearing wall. On upper or older floors, ask a builder or engineer first.
  • Use a purpose-built 72 × 18 stand. Plan how you'll move water: a 25% change is about 27 gallons.

General guidance, not structural advice.

Will a 125 gallon fit? Stand, doorway and floor

What stand does a 125 gallon need?

One with a top at least 72½ × 18½ in, the Aqueon footprint, rated for at least 1,400 lb. That is the higher of the makers' full weights; our own set-up estimate is 1,208 lb. The whole bottom frame must sit on the stand, with nothing hanging over the edge.

Will a 125 gallon fit through a door?

Usually, yes. Carried end-first, the doorway only has to clear the tank's narrowest side, 18½ in (47 cm), plus room for your hands. The 72½ in length only matters on tight turns in halls and stairs. Empty, it weighs about 177 lb (Aqueon).

Can my floor hold a 125 gallon?

Set up, it puts about 1,208 lb on 9.3 sq ft, or about 130 lb per square foot, before the stand. US homes are designed for 40 lb per square foot spread over a whole living-room floor (IRC Table R301.5), so a tank concentrates its weight. Over 1,000 lb, ask a structural engineer or builder before you fill it, especially upstairs or on an old or bouncy floor. That weight band is our rule of thumb, not a code limit. Check your exact setup.

Equipment for a 125 gallon

ItemStarting pointHow we got it
Heater319–532 W3–5 W per gallon of real water; use the higher end in a cold room.
Filter425–638 gphTurning over the water 4–6 times an hour
Sand, 2 in deep≈ 145 lbInside floor area × depth at about 1.6 kg per litre (estimate)
Light72 in (or 2 × 36 in)Match the tank length
Lid72 × 18 in (usually in sections)Check the maker's fit list

Rules of thumb, not guarantees. Fish choice, room temperature and filter type all change the right answer.

What a 125 gallon is good for

Six feet of length suits large, active fish and big schools that need swimming room. Check any mix in the stocking calculator.

How the numbers are worked out

Outside volume (gal) = L × W × H ÷ 231 Inside length/width = outside − 2 × glass Water height = H − glass (bottom) − gap below rim − substrate depth Water (gal) = inside L × inside W × water height ÷ 231 Water weight (lb) = gallons × 8.34 (fresh water) Substrate (lb) = inside L × inside W × depth × 0.016387 L/in³ × 1.6 kg/L × 2.2046

Worked examples

Aqueon 125, 1/2 in glass, 1 in gap, 2 in sand: inside 71.5 × 17.5 in, water height 23.125 − 0.5 − 1 − 2 = 19.62 in. 71.5 × 17.5 × 19.62 = 24,556 in³ ÷ 231 = 106.3 gal (402.4 L).

Marineland 125, 1/2 in glass, 1 in gap, 2 in sand: inside 71.56 × 17.72 in, water height 22.125 − 0.5 − 1 − 2 = 18.62 in. 71.56 × 17.72 × 18.62 = 23,616 in³ ÷ 231 = 102.2 gal (387.0 L).

Bare-bottom Aqueon 125, 1/2 in glass, 1 in gap, 0 in sand: inside 71.5 × 17.5 in, water height 23.125 − 0.5 − 1 − 0 = 21.62 in. 71.5 × 17.5 × 21.62 = 27,058 in³ ÷ 231 = 117.1 gal (443.4 L).

FAQ

What are the dimensions of a 125 gallon aquarium?

About 72 × 18 × 22 in (6 ft long). Aqueon lists 72½ × 18½ × 23⅛ in and Marineland 72 9/16 × 18 23/32 × 22⅛ in.

How much does a 125 gallon tank weigh full?

Aqueon lists 1,400 lb and Marineland 1,160 lb. Our estimate with 2 in of sand is about 1,208 lb before rock and the stand.

Is a 125 gallon tank 6 feet long?

Yes. Standard 125s are about 72 in long, which is why they need a 6 ft stand and light.

Can my floor hold a 125 gallon tank?

Many floors can, but it depends on the joists, span and position. At about 130 lb per square foot of footprint, get advice from a builder or engineer for upper or older floors. This is general guidance, not structural advice.

How many litres is a 125 gallon tank?

125 US gallons is 473 litres. Set up, expect about 387–443 litres of actual water.

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